Loch Garry

There has been some recent interest in the water levels and operating procedures at Loch Garry. This fact sheet was prepared on June 17th, 2011 to address public concern. The general public is encouraged to contact the RRCA if they have any questions about the water levels at Loch Garry, Middle Lake or Mill Pond.

Caring for Loch Garry

If residents have questions on how they can improve their impact on Loch Garry, the office of the Raisin Region Conservation Authority is ready to help. We can offer advice on properly maintaining septic systems; tips for reducing contaminated runoff into waterways; and help on implementing best practices towards ecosystem-friendly water-front living.

Background

Loch Garry is the first (and largest) lake of three which comprise the Garry River sub-watershed. The water from Loch Garry travels through Middle Lake to Mill Pond and then into the Delisle River. The proper management of all three lakes is critical to ensuring an adequate supply of drinking water for the Town of Alexandria. The Raisin Region Conservation Authority (RRCA) manages the water levels in the lakes by adjusting the dams at the outflows of the three lakes.

There are no rivers supplying water to Loch Garry. Water in the lake is supplied through the hydrologic cycle within the local drainage area. The water in Loch Garry comes from precipitation: snow melt and rain. When it rains, a portion of the water runs off the surrounding 16 square kilometers of land and enters the lake; another amount of water percolates through the ground and discharges into the lake.

Where does the water go?

Approximately 50% of the water leaves through evaporation. A small amount migrates naturally through the ground. The remaining water flows through the dam on Lakeshore Road. If more water leaves the lake than enters, the water levels drop. Conversely if more water enters the lake than leaves, the water levels rise.

The Garry River Watershed

The entire watershed area is relatively small, only 34 square kilometers. Rain and snow falling inside the red line below is the primary source of new water in the lakes.

Management of the Lakes

The three lakes work as a system: an adjustment to the outflow of one lake will affect the water levels of all three lakes. Water leaving Mill Pond enters the Delisle River just downstream of the Town of Alexandria. The RRCA’s primary concerns for managing the lake system are:

Ensure sufficient water supply for the Town of Alexandria

Ensure a sufficient water supply to the Delisle River (The Ministry of the Environment compels a minimum specified amount of flow every day)

Flood prevention and protection (around the lakes and dams)

Environmental considerations

Recreational considerations

Operational Considerations

The RRCA sets and reviews operational guidelines to ensure the lakes are operated to meet the management objectives, while considering the following:

The majority of outflow through the dams is during the spring months (the Spring Freshet)

The majority of evaporation occurs during the summer months

Snowfall is not usually available to replenish the lake levels during the winter months.

The lake levels must be constantly monitored to ensure they are low enough to prevent flooding in the event of significant precipitation, and at the same time high enough to sustain extended periods without rain.

For Loch Garry, the operational guidelines include making adjustments to the water level throughout the year. During the summer months, the target water level elevation is 89.1 masl (meters above sea level) *. The RRCA strives to maintain water levels above a minimum of 88.9 masl during the summer.

*Water Elevations are generally measured to a standardized datum. Meters above Sea Level (masl) is a common datum. A water level of 88.9 masl does not mean that the water is 88.9 meters deep. To compute the depth, you would need to know the elevation of the bottom of the lake, and subtract it from the water level elevation.

Historic Records

The RRCA maintains detailed records of water levels in all of the lakes. The minimum levels since 2003 for the period between June 1 and August 31 are listed below.

Year

Minimum Level (masl)
June 1 to August 31

2010

89.082

2009

89.139

2008

89.142

2007

89.021

2006

89.170

2005

88.863

2004

88.911

2003

89.010

Current Conditions (as of June 17th, 2011)

On May 30th, 2011 the water level at Loch Garry was 89.100 masl. We are obliged not exceed this level.

On June 2nd, 2011 the water level at Loch Garry was 89.080 masl. After consulting the long-range precipitation forecast, the dam was adjusted so that no flow would pass over.

On June 17th, 2011 the water level at Loch Garry was 89.034 masl.

The RRCA has not let water out of Loch Garry since June 2nd. The decrease in water levels are through natural causes (i.e. evaporation).

The current level is in fact the lowest of the year, but not the lowest in recent history for this time period.

The current level is still above the operational guideline of 88.9 masl.

A drop of 14 inches (0.356 meters) over 10 days has not occurred this year.

Over the last 10 days, water levels have dropped 0.041 meters (1.6 inches). Again, this drop is due to evaporation.

Water levels have been slowly dropping since May 25th. Since then the water level has dropped about 4 inches in 29 days